A light emitting device using a light emitting diode (LED) is widely used for optical devices such as indoor, outdoor, stationary, mobile display devices, display lamps, various switches, signal devices, general lightings and so on. Among the light emitting devices using LEDs, as a device suitable for display devices that display various character strings, geometrical figures and patterns and the like and a display lamp, a transparent light emitting device is known in which a plurality of LEDs are arranged between two transparent substrates. Using a flexible substrate made of a transparent resin as the transparent substrate eases the constraint on the attachment surface of the light emitting device as the display device or the display lamp, resulting in improved convenience and availability of the transparent light emitting device.
The transparent light emitting device has a structure in which a plurality of LED chips are arranged, for example, between a first transparent insulating substrate having a first conductive circuit layer and a second transparent insulating substrate having a second conductive circuit layer. Each of the plurality of LED chips has a pair of electrodes. One of the electrodes is electrically connected to the first conductive circuit layer, and the other electrode is electrically connected to the second conductive circuit layer. The plural LED chips are arranged with a certain interval therebetween. In a space between the first and second transparent insulating substrates occurring based on the arrangement interval between the plural LED chips, a transparent insulator made of a transparent resin or the like having electric insulation property and flexibility is filled. In other words, the LED chips are arranged in through holes provided in the transparent insulator.
The electrical connection between the electrodes of the LED chips and the conductive circuit layers in the transparent light emitting device is sometimes made, for example, by vacuum thermocompression bonding a stack composed of the first transparent insulating substrate, a transparent insulating resin sheet in which the LED chips are arranged in the through holes, and the second transparent insulating substrate. The electrodes of the LED chips and the conductive circuit layers are sometimes bonded with each other with a conductive adhesive. It is also under discussion that thermocompression bonding is performed with a hot melt adhesive sheet, to which the LED chips are fixed, sandwiched between upper and lower insulating substrates having the conductive circuit layers to embed the LED chips in the adhesive sheet, thereby performing the bonding of the upper and lower insulating substrates and the electrical connection between the electrodes of the LED chips and the conductive circuit layers at the same time.
However, it is impossible, in any case, to sufficiently increase the electrical connection between the electrodes of the LED chips and the conductive circuit layers and its reliability. For example, it is under discussion that, in the case of vacuum thermocompression bonding the stack composed of the first transparent insulating substrate, the transparent insulating resin sheet, and the second transparent insulating substrate, the thickness of the transparent insulating resin sheet after the thermocompression bonding (the thickness of the transparent insulator) is made smaller than the thickness of the LED chip to press the conductive circuit layers against the electrodes of the LED chips so as to bring them into contact. However, depending on the material and the thickness of the transparent insulator or the arrangement interval of the LED chips, it is impossible to reliably electrically connect the electrodes and the conductive circuit layers. Hence, a technique that increases the electrical connection between the conductive circuit layers and the electrodes of the LED chips and its reliability with high reproducibility, is required.